I have found through trial and error that the best batteries you can buy are usually the ones custom made.
Lets take my Treo 650 for example.
The test: Playing metaviewsoft's ZQauke, and using PXA Clocker to boost my CPU to 429mhz
The default 1500mah battery that it comes with will usually last about 2 hours of game play
With a Cameron Sino (heron known as CS) 3200mah battery, I get about 3 and half hours of game play.
With a Seidio 3200mah battery I get a little over 4 hours of game play.
As you can see the Seidio battery is the most efficient by commercial standards.
Its surprising to note that the CS battery has a shorting life span than the Seidio battery, even though both are rated at 3200mah.
So I conducted a test on 5 CS 3200mah batteries and did a max discharge on all of them. They all toped of at about 2850mah. IMO I would not buy from CS.
As for me I use a custom Lithium polymer battery. They have  more power in a smaller space because they don't have a clunky metal case like Li-Ion does.
I was able to get a 4100mah Li-Po battery custom made to fit in the Seidio extended battery compartment. and now I can get roughly about 5.5 to 6 hours of continuous  game
play. I was lucky enough to make a custom sized battery but most sites that do this will force you to by a minimum amount. Usually around 5.